The present invention generally relates to semiconductor laser control apparatuses, and more particularly to a semiconductor laser control apparatus for controlling a semiconductor laser which is used as a laser light source in an optical image forming apparatus including a laser printer, a laser facsimile machine and the like.
As is well known, a semiconductor laser control apparatus is used to control an optical output of a semiconductor laser which is used as a light source of an optical image forming apparatus. A known semiconductor laser control apparatus detects the optical output of the semiconductor laser and compares the detected optical output with a reference value. An up-down counter counts up or down depending on a result of the comparison, and a driving current dependent on a counted value of the up-down counter is supplied to the semiconductor laser.
However, in this semiconductor laser control apparatus, the optical output of the semiconductor laser may deviate due to a thermal coupling of the semiconductor laser. For this reason, even when the optical output of the semiconductor laser is detected and the driving current supplied to the semiconductor laser is adjusted to a constant value, the optical output at a time when the semiconductor laser turns ON may become larger than the optical output determined by the driving current and then settle to a set value with a certain time constant.
In this case, when a modulation signal shown in FIG. 1(A) turns the semiconductor laser ON and OFF and a driving current shown in FIG. 1(B) is supplied to the semiconductor laser, the optical output of the semiconductor laser deviates in a vicinity of the rising edge as shown in FIG. 1(C). Such a deviation of the optical output causes an inconsistent image density and may make it impossible to reproduce with a high fidelity halftone images on a laser printer and the like which uses the semiconductor laser as the laser light source.
Accordingly, it is possible to consider controlling the optical output of the semiconductor laser by a conceivable semiconductor laser control apparatus shown in FIG. 2. This conceivable semiconductor laser control apparatus has a driving circuit 2 for supplying a driving current to a semiconductor laser 1, a rise timing correction circuit 3 coupled to the driving circuit 2, and a differentiating circuit 4 coupled between the driving circuit 2 and the rise timing correction circuit 3. A modulation signal is supplied to the rise timing correction circuit 3 through a terminal 5. The rise timing correction circuit 3 in combination with the differentiating circuit 4 act as a correcting means for supplying to the semiconductor laser 1 a correction current having a predetermined time constant when the semiconductor laser 1 is ON.
The rise timing correction circuit 3 may have a circuit construction shown in FIG. 3. In FIG. 3, the modulation signal from the terminal 5 is delayed in a delay line 3a and supplied to a terminal 31. On the other hand, the modulation signal from the terminal 5 is supplied as it is to a terminal 32. A delayed modulation signal from the terminal 31 is supplied to the driving circuit 2 shown in FIG. 2 as a phase delayed signal so as to adjust a timing with which the correction is made with respect to the optical output of the semiconductor laser 1. The undelayed modulation signal from the terminal 32 is supplied to the differentiating circuit 4. Hence, the delay line 3a is necessary only when the correction timing is required.
The differentiating circuit 4 shown in FIG. 2 differentiates the modulation signal which is to be supplied to the driving circuit 2, so as to supply a predetermined correction current to the semiconductor laser 1 when the semiconductor laser 1 is ON and correct the deviation in the optical output of the semiconductor laser 1.
In a case where the semiconductor laser 1 the optical output of which is to be corrected has an output characteristic with n kinds of time constants, the differentiating circuit 4 is constituted by n differentiators 4.sub.1 through 4.sub.n which are coupled in parallel as shown in FIG. 4. The differentiators 4.sub.1 through 4.sub.n respectively provide correction quantities respectively corresponding to the time constants of the output characteristic of the semiconductor laser 1. In FIG. 4, those parts which are essentially the same as those corresponding parts in FIG. 2 are designated by the same reference numerals, and a description thereof will be omitted.
The differentiating circuit 4 and the differentiators 4.sub.1 through 4.sub.n may have a known simple construction including a resistor and a capacitor or may additionally have an operational amplifier.
According to the conceivable semiconductor laser control apparatuses shown in FIGS. 2 and 4, the phase of the modulation signal received at the terminal 5 is adjusted in the rise timing correction circuit 3 so that the modulation signals supplied to the driving circuit 2 and the differentiating circuit 3 have an appropriate phase relationship.
But according to such conceivable semiconductor laser control apparatuses, the deviation of the optical output of the semiconductor laser 1 is suppressed by setting the time constant and the current quantity (quantity corresponding to n=1, n=2 and n=3 in FIG. 5A which shows the optical output of the semiconductor laser 1) to one of correction currents CC1, CC2 and CC3 respectively shown in FIGS. 5B, 5C and 5D so as to correct the deviation of the optical output of the semiconductor laser 1 when no correction is made. Hence, a rising edge of a driving current I.sub.op with respect to the semiconductor laser 1 after the correction is slow with respect to a rising edge of the modulation signal, as shown in FIG. 5E. When the correction quantity is large, a rise start time corresponding to a time when the semiconductor laser 1 turns ON responsive to a driving current with which a predetermined optical output is obtained becomes delayed by a delay time DT as indicated by a one-dot chain line in FIG. 5E. In this case, with respect to the operating time of the semiconductor laser 1 responsive to the modulation signal shown in FIG. 6(A), a time in which the driving current Iop after the correction shown in FIG. 6(B) is supplied to the semiconductor laser 1 becomes shortened by the effect of the delay time DT. FIG. 6(C) shows the optical output of the semiconductor laser 1 after the correction. Therefore, the time in which the semiconductor laser 1 should operate responsive to the modulation signal and the time in which the driving current I.sub.op after the correction is actually supplied to the semiconductor laser 1 do not coincide. As a result, there are problems in that the duty characteristic of the optical output of the semiconductor laser 1 becomes deteriorated and it no longer becomes possible to reproduce the image with a high fidelity.